This continuation grant application requests support for part of a diversified, cell biological investigation of the mechanism of cell cleavage (division) in animal cells. For the coming year, the principal cell type to be studied is the sea urchin egg and special emphasis will be given to the brief period just before cleavage when the "cleavage stimulus" appears. Its nature is completely unknown. Contractile ring microfilaments and mitotic apparatus microtubules will be studied by electron microscopy and immunocytochemistry. Specific studies include a 3-D analysis of microtubules in the vicinity of the incipient cleavage furrow; changing patterns of structural and biochemical elements in the cell cortex by polarization microscopy, immunofluorescence microscopy, and electrokinetic cell behavior; an analysis of an unusual, unequal cell division. BIBLIOGRAPHIC REFERENCE: Schroeder, T.E. 1976. Actin in dividing cells: evidence for its role in cleavage but not mitosis. In Cell Motility (Eds. Goldman, R. et al.) Cold Spring Harbor Laboratories. New York.